1. Field of the Invention
The present invention relates to an optical data recording medium having two or more data recording layers, and to a manufacturing method for this optical data recording medium.
2. Description of Related Art
Optical data recording media such as optical discs are quickly becoming the information recording medium of choice for a wide range of applications. Common types of optical discs include read-only, incrementally writable (e.g., multisession), and rewritable media. Compact Discs (CDs) used for distributing packaged audio content and Laser Discs for recording video data (such as movies) are typical read-only optical discs. Multisession CDs for storing text files and still images, for example, are typical of incrementally writable media. Rewritable media include a variety of recordable discs for storing personal computer data files and other types of content.
One advantage of optical media is that optical discs enable a significantly higher data recording density than the magnetic data recording density of conventional magnetic disks. Optical media can also be written and read without the read/write head physically contacting the disc. Durability with respect to vibration and electromagnetic fields can therefore be improved and disc life can be increased.
Optical discs today typically have a data recording layer disposed to a 1.2 mm thick transparent resin substrate protected by an overcoat, or a data recording layer disposed to one or both of two 0.6 mm thick transparent resin substrates that are then bonded together to form the optical disc.
High capacity Digital Versatile Discs (DVD) have also been developed for recording movies and other types of video content in addition to audio. A short wavelength laser and an objective lens with a large numerical aperture (NA) are used to achieve high density DVD media. A short wavelength and high NA, however, decrease the tolerance for tilt, i.e., an inclination of the disc to the incidence angle of the laser beam.
One effective way to increase tilt tolerance is to decrease substrate thickness. DVD, for example, use a 650 nm wavelength laser, 0.60 NA, and 0.6 mm substrate thickness. Because a 0.6 mm thick resin substrate is mechanically weak, two substrates are bonded together with the data recording surfaces to the inside. This bonded substrate construction has also been used to make DVD discs with two recording (playback) layers on a single side of the disc. DVD discs having two recording layers on a single side form a transparent reflective layer of gold and silicon, for example, on the data recording surface of one of the two substrates, have a conventional reflective layer of aluminum, for example, on the other data recording surface, and are bonded with the data recording layers on the inside. During playback both data recording layers are read from the side of the disc to which the transparent reflective layer was formed.
Rewritable DVD media having a two-layer construction but having a rewritable thin-film recording layer on the data recording surface instead of a metallic reflective layer have also been proposed.
Data transmission volume has also increased in recent years with the advent of HDTV broadcasting, for example, thus necessitating a further increase in the data recording density of recording media. One way to increase the recording density is to shorten the wavelength of the read/write laser and increase the NA of the objective lens. In this case, reducing the thickness of the substrate on the read/write side to which the laser beam is incident can reduce the effect of aberrations on the laser spot and increase tolerance to disc tilt. It has therefore been proposed to set the substrate thickness on the read/write side of the disc to approximately 0.1 mm and use a 0.85 NA and a 400 nm laser. (See, for example, Japanese Laid-Open Patent Publication No. 2001-357571.) In this case, variation in the thickness of the read/write side substrate is preferably 5% or less considering the effect on spherical aberration and focusing the read/write beam.
Multilayer discs with two or more data recording layers in a single disc have been designed to achieve a high data density. With optical discs having two or more data recording layers (referred to below as a “multilayer discs”) the laser is emitted to each of the multiple data recording layers from a single side of the disc to read data from each layer. This means that as seen from the laser-emission side of the disc information on any data recording layer other than the first layer is read through any intermediate layers located between adjacent recording layers. These intermediate layers are thus the transmission path of the laser beam used to read the data. The thickness of these intermediate layers is therefore as uniform as possible.
The intermediate layers disposed between two data recording layers in such multilayer discs are often formed by spin-coating the resin material. For example, in an optical disc with two data recording layers a resin material is applied by spin coating to the first data recording layer formed on the substrate and then cured to form an intermediate layer.
Unfortunately, it is difficult with conventional spin coating techniques to apply the resin material with a uniform thickness from the inside circumference side to the outside circumference side of the disc. More particularly, there is a tendency for the applied resin material to be thin at the inside circumference and thick at the outside circumference side of the disc.
Multilayer discs as described above have very low tolerance to variation in the thickness of the layers that must be passed by the read/write laser. More specifically, a maximum deviation of only +/−2 μm is allowed in the thickness of the approximately 0.1 mm thick layers passed by the laser. Furthermore, because the read/write side substrate is a mere 0.3 mm thick, it cannot be formed by injection molding, and it is difficult to form these transparent layers to such stringent thickness specifications.
It is also difficult to injection mold data recording layers with grooves or pits and lands for a multilayer discs with layers 0.3 mm thick or less. The 2P method is well known as a non-injection molding method for forming grooves or pits and lands, but it is not easy to satisfy this strict tolerance for thickness deviation with the 2P method.
An object of the present invention is therefore to provide a manufacturing method for an optical data recording medium having two or more data recording layers whereby thickness deviation can be suppressed and an intermediate layer of uniform thickness can be formed between each of the adjacent two or more data recording layers.